Receiver systems typically receive an input signal and generate one or more output signals by performing various processing of the input signal. Different receiver system designs offer varying performance characteristics depending on the specific application of the receiver system. Example performance characteristics include frequency selectivity, power consumption of the receiver system, noise figure, one decibel compression point (P1dB), third-order intercept point (IP3), dynamic range, and immunity to signal corruption within the receiver system.
Conventional receiver systems provide amplification and frequency translation of an input signal while dissipating significant energy from the available power source, especially components appearing early in the RF signal path (e.g., closest to the antenna). Such receivers use active components, such as low noise amplifiers (LNA) and/or current-commutating mixers, in the RF signal path to amplify and/or translate the frequency of the input signals provided by the antenna. The critical performance metrics for these active components, such as noise figure (NF) and linearity (often specified by P1db and/or IP3), have fundamental limits dependent on the power consumption of the active component and are generally improved by increasing the bias current and/or available voltage headroom. These power-performance tradeoffs lead to high current consumption when high performance is desired or, equivalently, poor performance when low-power is desired.
In contrast, passive components (e.g., inductors, capacitors, resistors and switches) offer high intrinsic linearity and consume little or no power. Some passive networks, such as LC resonators constructed with high quality factor components, can achieve substantial voltage gain with low loss and hence, low noise figure. In addition, highly linear, low noise frequency translation can be achieved using passive switches and capacitors (e.g., a passive mixer). In a passive mixer, the signal path remains passive while minimal power is spent activating and deactivating the constituent switches. Furthermore, advances in process technology may bring lower resistance switches that require less energy to activate and deactivate, improving the power/performance tradeoffs in the passive mixer. Thus, passive circuits offer an opportunity to achieve equivalent functionality to conventional active components in a receiver front-end with dramatically lower power consumption.
A known wireless transceiver using a substantially passive RF receiver signal path was described by the authors in IEEE Journal of Solid-State Circuits, Vol. 4, No. 12, December 2006, “Low-Power 2.4-GHz Transceiver With Passive RX Front-End and 400-mV Supply”, Ben W. Cook et al. This publication discloses a basic receiver including a passive voltage gain network driven by an external antenna and directly followed by a passive switching mixer providing frequency translation to baseband. This receiver provided signal amplification and frequency translation while consuming very little power. Due to the relaxed performance requirements of the intended application, the disclosed receiver was conceived so as to minimize power consumption and support operation from very low supply voltages. As a result, and as pointed out in the publication, robustness and performance were not critical to the intended application, whereas minimizing complexity and power consumption were the focal points of that work.
The passive voltage gain network disclosed in the publication had a fixed center frequency and a fixed voltage gain. The lack of gain control prevented the receiver from being able to accommodate a wide range of input signals, as is generally required for wireless data applications. In the published work, the passive switching mixer was driven directly by a free-running voltage controlled oscillator (VCO). Thus, the sampling frequency of the receiver was neither well-defined nor stabilized and the receiver was not able to automatically tune to and receive signals from different RF communication channels. Secondly, since there was no isolation between the VCO and the mixer, the sampling frequency of the mixer was susceptible to corruption by pulling due to the coupling of RF input signals. Furthermore, the sinusoidal shape of the VCO waveforms driving the mixer, coupled with a requirement of non-overlap between driving phases, degraded the linearity and noise performance achieved by the receiver. Lastly, the passive switching mixer utilized in the publication could not achieve voltage gain greater than 0 dB, thus limiting the overall RF gain and thereby impairing the achievable sensitivity for a given power budget.
Many applications favor a receiver system that consumes as little energy as possible. For example, handheld wireless data devices generally favor long battery lifetime yet must maintain adequate performance to comply with a given wireless communication standard. In these applications, a receiver system that meets performance specifications and provides signal amplification and frequency translation using low power passive circuitry, rather than current consuming active components, is desirable.
Throughout the description, similar reference numbers may be used to identify similar elements.